1. Field of the Invention
The present invention relates to an isolated switching power supply apparatus including a compound transformer that includes a plurality of secondary windings and that utilizes a leakage inductance and does not require an external choke coil.
2. Description of the Related Art
An output current smoothing low-pass filter including an external inductor and a smoothing capacitor has typically been used as an isolated switching power supply apparatus. It is known that a leakage inductance caused in a secondary side of a transformer is used in order to decrease an inductance value of the external inductor. Also known is a technique in which the leakage inductance is intentionally increased by reducing the degree of magnetic coupling of the transformer.
In accordance with Japanese Unexamined Patent Application Publication No. 2001-218457, a leakage inductance is intentionally produced by arranging a core gap GAP4 in a core of an isolated transformer T2 as illustrated in FIGS. 1A and 1B, such that an external inductor L is miniaturized.
In accordance with Japanese Unexamined Patent Application Publication No. 2003-533163 as illustrated in FIG. 2, an isolated transformer including a primary winding and a second winding, each having a number of turns N, is provided, and a leakage inductance produced by the isolated transformer is intentionally increased to be used as an inductor L2.
As illustrated in FIG. 3, Japanese Unexamined Patent Application Publication No. 2003-533163 discloses a technique in which a leakage inductance of a tertiary winding 5c of a transformer T1 is increased by low-coupling a primary winding 5a to the tertiary winding 5c so as to eliminate the need for any external inductors.
As illustrated in FIG. 4, Japanese Unexamined Patent Application Publication No. 2007-43858 discloses a technique in which a leakage inductance L1 of a transformer T is used so as to eliminate the need for an external choke coil.
The isolated switching power supply apparatuses described above suffer from a basic problem. More specifically, a leakage inductance of a transformer may be used for an output current smoothing inductor. However, a leakage inductance only has a small inductance value and provides relatively low output current smoothing performance. A large output current ripple results. The output current ripple becomes large particularly when a difference between a voltage produced in a secondary winding during an on period of a primary switching element and a voltage produced in the secondary winding during an off period of the primary switching element increases in response to a change in an on time ratio of the primary switching element in order to control the output voltage.
In addition, in an attempt to increase the value of a leakage inductance, the degree of magnetic coupling of a transformer may be intentionally decreased by arranging an air gap in the core of the transformer. Such an arrangement causes an energy transfer inductance value to be decreased, which leads to a bulky transformer.
Furthermore, if the leakage inductance of the transformer is used, or if a choke coil is used for an external inductor, these components include a magnetic body as a core. A direct-current magnetic flux is produced in the magnetic body in response to an output current. In order to prevent magnetic saturation, the use of a bulky transformer or a bulky choke coil is unavoidable.